The resistance engendered in mice by sensitization with nonspecific and specific immunogens will be studied employing Friend virus (FV) and malignant melanoma cells (Mb16) to induce disease. It is evident that nonspecific resistance to infection with FV can be induced and studied by administration of both the immunogen and the pathogenic agent by the intraperitoneal (ip) route and that washed cells of the spleens of mice with Friend disease (FD) will produce disease when inoculated ip into mice. This suggests that the factors essential for control of FD are present in the peritoneal cavity of specifically and nonspecifically immunized mice. Attempts will be made to determine the difference between the mechanisms involved in animals immunized by subcutaneous injection of live virus and by ip injection of Mycobacterium bovis (BCG) or products of this organism. The ip route will be employed for administration of Friend virus or cells. Studies will be directed primarily to studies of the peritoneal exudates by the in vitro methods of Hibbs, Alexander, Remington, Granger and others. It should be possible to determine and compare the relative importance of antibodies, macrophages and lymphocytes utilizing these techniques. Studies of malignant melanomatosis in mice to whom cells are administered iv will be continued.